A scroll-type fluid machine of this kind, for example, a scroll-type compressor, is provided with a scroll unit for carrying out a series of processes including the suction, compression, and discharge of a refrigerant. Specifically, the unit comprises fixed and movable scrolls that are engaged with each other. The movable scroll makes a rotating movement around the fixed scroll. Therefore, the capacity of a space formed by each of the scrolls is reduced, and the above-mentioned processes are carried out.
In the compression process, a high-pressure space is produced in the scroll unit due to the discharge pressure of the refrigerant. This pressure acts as thrust load from the front side of the movable scroll toward the rear side thereof. This load moves the movable scroll in the direction of moving away from the fixed scroll. The rear side of the movable scroll is supported on a surface oriented to the fixed scroll in order to perform the above-mentioned processes without fail. In other words, a supporting reaction force counteracting the thrust load acts on the rear side of the movable scroll so as to move the movable scroll in the direction of approaching the fixed scroll. As a result, the front side of the movable scroll abrades away due to friction against the fixed scroll, which degrades the performance of the scroll unit.
Therefore, there has been disclosed a technology of reducing the thrust load by escaping the refrigerant acting on the front side of the movable scroll to the rear side through the inside of the movable scroll (se Unexamined Japanese Patent Publication Nos. 2000-136782, 2000-249086, and 2000-352386).
Since the above-mentioned processes are carried out in the scroll unit, the refrigerant pressure acting on the front side of the movable scroll constantly fluctuates until reaching the discharge pressure.
To be concrete, as disclosed in the conventional technology, when the refrigerant in the process of being compressed is escaped to the rear side of the movable scroll through its inside, the pressure acting on the rear side also fluctuates. Moreover, the refrigerant acting on the front side of the movable scroll is not always immediately delivered to the rear side of the movable scroll. This arouses concern that the thrust load cannot be effectively offset. That is, the above-described technologies have not yet solved the issue of reducing the thrust load.
In recent years, a refrigeration circuit using a refrigerant having a small global warming potential (GWP) value has been developed in consideration to global environment. An example of this kind of refrigerant is natural CO2 (carbon dioxide) gas. As this refrigerant has high working pressure, it is especially requested in this case to reduce the thrust load.
In order to use a CO2 refrigerant having high working pressure, it is preferable that the scroll unit have both simplicity and rigidity. It should be noted that, for example, the structure in which a communication hole is formed in the movable scroll, in which there is provided a check valve for preventing a counter flow from the rear side of the movable scroll to the front side thereof, in which an elastic member is provided to the rear side of the movable scroll, or the like, potentially becomes a hindrance to the above-mentioned processes performed by the scroll unit. Especially in case that the communication hole is formed in the movable scroll, it should be noted that compression efficiency is lowered when the refrigerant acting on the front side of the movable scroll moves to the rear side.